Magnetic thin films and multilayers play a key role in various types of magnetic storage devices such as a magnetic hard disk (HDD) drive, Magnetic Random Access Memory (MRAM), spin torque oscillator (STO), and magnetic domain wall devices. In order to develop and optimize such devices, monitoring and characterization of magnetic thin film stacks is essential. A variety of different magnetic characterization techniques must be used to determine all the essential magnetic parameters such as crystalline anisotropy, surface or interface anisotropy, magnetization saturation (Ms), damping constant (α), gyromagnetic ratio (γ), inhomogeneous broadening (L0), resistance x area product (RA), and magnetoresistive ratio (MR).
FMR techniques are well suited to measure anisotropy fields, as well as the gyromagnetic ratio γ, and the damping constant α of magnetic films and multilayers in extended unpatterned films or over an area comprising a large array of sub-micron patterned structures. The resonance frequency fR of a ferromagnetic film is given by the so-called Kittel formula shown in equation (1) below where HR is the resonance field applied perpendicular to the plane of the film, HK is the effective anisotropy field which includes structural, surface, and magnetostatic contributions, and γ is the gyromagnetic ratio.2πfR=γ(HR+HK)  (Eq. 1)
A FMR experiment comprises probing the magnetic system (thin film, multilayer stack, or structured device) with a combination of microwave excitation and a quasi-static magnetic field. FMR data is obtained by either sweeping the magnetic field at a constant microwave frequency, or by sweeping the frequency at a constant field. When the ferromagnetic resonance condition is achieved, it may be detected by an enhanced absorption of the microwave (RF signal) by the ferromagnetic sample. The absorption is at a maximum at a specific frequency corresponding to the resonance frequency (fR) of the sample where fR depends on the static field applied to the sample as well as its magnetic properties. Thus, resonance (FMR) conditions are defined using pairs of magnetic field and microwave frequency values (HR, fR).
Although conventional FMR experiments were done by placing a small sample in a resonant cavity between the poles of an electromagnet, waveguide based techniques, which have been developed during the past decade, are especially well suited to analyze film geometry. In particular, the wafer under test (WUT) is placed in contact with a waveguide transmission line (WGTL) that may be in the form of a grounded coplanar waveguide (GCPWG), coplanar waveguide (CPWG), co-axial waveguide (CWG), stripline (SL), or a microstrip (MS). The WGTL is used both to transmit microwaves to the sample, and to detect FMR absorption as a function of the applied magnetic field and microwave frequency.
Referring to FIG. 1A, a schematic depiction is shown where output voltages are plotted as a function of a variable magnetic field at constant microwave frequency using five different values (f1-f5) of microwave frequency. The center and width of the Lorentzian peaks is extracted from the data as a function of the excitation microwave frequency. As mentioned previously, the center field is the resonance field (HR), which is related to the excitation microwave frequency following the Kittel formula that is rewritten in a slightly different form in equation (2) below where h is the Planck constant, g is the Lande factor and μB is the Bohr magneton.HR(f)=[h/(g×μB)]×f−HK  (Eq. 2)
The variation of HR with microwave frequency is shown in FIG. 1B where each of the points along curve 21 is derived from one of the Lorentzian shaped peaks Hr1-Hr5 in FIG. 1A. As indicated by equation (2), the extrapolation of the data to f=0 gives the value of the effective anisotropy field HK.
The linewidth L of the resonance peak is the width at half amplitude ΔH of the resonance peak and is related to dissipative processes involved in magnetization dynamics as well as to possible distributions of different magnetic thin film parameters such as HK or Ms. The linewidth depends on the excitation frequency and the dimensionless Gilbert damping constant α according to equation (3) below where L0 is an inhomogeneous broadening. By fitting HR and L with respect to the excitation frequency fR, HK as well as α, L0 and g may be derived.L(f)=(2hα/(g×μB))f+L0  (Eq. 3)
A vector network analyzer (VNA) for detecting FMR in thin CoFe and CoFeB films on a coplanar waveguide is described by C. Bilzer et al. in “Vector network analyzer ferromagnetic resonance of thin films on coplanar waveguides: Comparison of different evaluation methods” in J. of Applied Physics 101, 074505 (2007), and in “Open-Circuit One-Port Network Analyzer Ferromagnetic Resonance” in IEEE Trans. Magn., Vol. 44, No. 11, p. 3265 (2008). In these experiments, the planar WGTL is typically attached to radiofrequency (RF) connectors by microwave electrical probes and placed between the poles of an electromagnet. Thus, given the size of the WGTL and the size of the gap of typical electromagnets, only small samples (normally <1 inch in diameter) can be measured. Accordingly, wafers typically used in the microelectronics industry (having diameters of 6, 8, 12 inches or more) can only be measured with this FMR technique if they are cut into small coupons.
Since conventional FMR techniques are destructive, time consuming, and limited to measuring small pieces of a wafer, they are undesirable to an extent that prevents wide acceptance of FMR as a characterization tool in the magnetic data storage industry. An improved FMR measurement system and technique is needed that enables fully automated measurements on whole wafers for faster throughput and lower cost. Moreover, the improved FMR system should be constructed from commercially available parts.